Hotmelt adhesives are an important adhesive class for many applications, for example in the automotive sector, in the packaging sector, in furniture production or in textile bonding. They are solid at room temperature and are melted by heating, and are applied to the substrate in substance at elevated temperature. In the course of cooling, they solidify again and thus ensure a firm adhesive bond after only a short time.
A subgroup of the hotmelt adhesives is that of reactive hotmelt adhesives which, after application, additionally crosslink and thus cure irreversibly to form a thermoset. As compared with the non-crosslinking, purely physically curing thermoplastic hotmelt adhesives, the additional chemical curing leads to a higher stability of the adhesive bond. Reactive hotmelt adhesives are particularly suitable for bonds in the automotive and transport sector and the packaging sector, and in the construction industry, textile industry and the wood-processing industry.
A preferred example of reactive hotmelt adhesives is one-pack moisture-curing hotmelt adhesives. These are generally isocyanate-functionalized polymers obtainable by reacting polyols or polyol mixtures with an excess of polyisocyanates.
The polyols may be any desired polymer bearing hydroxyl groups. Examples of these include polycarbonate diols, hydroxyl-terminated polybutadienes, polyethers or polyhydroxyacrylates or polyhydroxymethacrylates. Particular preference is given to polyester polyols. These can be prepared, for example, via the ring-opening polymerization of lactones and preferably via the solvent-free melt condensation of polyols with polycarboxylic acids or derivatives thereof. Preference is given to linear bifunctional polyester polyols formed from diols and dicarboxylic acids, diesters and/or dianhydrides. Reactive hotmelt adhesives based on polyester polyols exhibit good adhesion to a multitude of substrates, since the ester groups can bind to the substrate surface via hydrogen bonds. In addition, it is possible to adjust the polymer properties and hence also the adhesive properties over a wide range via the nature and ratio of the comonomers.
In the production of the aforementioned systems, it should be ensured that the polymers obtained have minimum VOC values. VOC or VOCs (volatile organic compound(s)) is the collective term for organic, i.e. carbon-containing, substances which evaporate readily and are therefore volatile.
A disadvantage is that several side reactions can occur in the course of melt condensation of diols with dicarboxylic acids, diesters and/or dianhydrides. One of these is the formation of cyclic dimers by ring closure of a diol molecule with a dicarboxylic acid derivative. The cyclic dimers form via an equilibrium reaction from linear dimers, which form in turn by transesterification reactions at the chain ends of the linear polyester chains (cf. EP 1571171). This means that a proportion of cyclic dimers is always present as well as the linear polyester. Depending on the size and stability of the ring, the typical proportion is up to 1% by weight (cf. U.S. Pat. No. 5,712,320).
The volatility of the cycles depends on the ring size and hence depends on the nature of the monomer units used. For example, adipic acid and diethylene glycol form a cyclic diethylene glycol adipate composed of a total of 13 ring atoms which sublimes readily (cf. U.S. Pat. No. 5,712,320). Since the cyclic dimers do not have any hydroxyl functionality, they do not react with isocyanate groups and are not incorporated into the polyurethane network. The cycles can therefore evaporate off or migrate out of the cured adhesive. This is problematic particularly in the case of bonds in motor vehicle interiors and in the food packaging sector. In the case of food packaging, the volatile constituents can migrate into the food and alter the taste or damage the food in such a way as to constitute a health concern. In the automotive sector, the volatile constituents accumulate in the breathable air within the passenger cell and can cause an unpleasant odour. These may be of concern to health. It is particularly critical when the outgassing substances are deposited on cold surfaces such as the windscreen and lead to visual impairment. This effect is generally referred to as “fogging”.
For the reasons mentioned above, for applications in the automotive interior sector, there is a demand for components and adhesives, sealants and coatings materials having low VOC and fogging values, which are generally measured by the industrial standard VDA 278. The standard defines the test conditions for determination of the emission values at particular temperatures. According to the limits required for cured adhesives and sealants of defined layer thickness, the VOC value, which is the proportion of volatile organic substances emitted as gases at 90° C. within 30 minutes, must not exceed 100 μg/g. In addition, what is called the FOG value, which is a measure of the amount of organic emissions at 120° C. within 30 minutes, must not be higher than 250 μg/g.
The literature, for example patent applications DE 19528539 and DE 19601410, JP 2004107457 and EP 1481998, describes various attempts to remove volatile cycles by distillation at elevated temperatures and under reduced pressure. If, for example, a polyester melt, after the condensation, is cooled down rapidly to temperatures below the softening point, it is possible to reduce the proportion of cyclic components. However, as soon as the polyester is melted again, the concentration of dimeric cycles rises again, since the cyclic components reform as a result of the equilibrium reaction.
Cured polyurethane hotmelt adhesives based on amorphous solid or liquid polyester polyol mixtures in particular combinations satisfy the limits of the automotive industry according to VDA 278. In contrast, cured formulations which, in addition to amorphous polyester polyols, comprise solid, crystalline or semicrystalline polyester polyols that are customary on the market have excessively high VOC and FOG values and do not satisfy the limits according to VDA 278. The reason is that the monomer units used customarily for the synthesis of crystalline polyesters form dimeric cycles which are volatile in the range relevant for VDA 278. These include, in particular, the dimers formed from aliphatic diols and dicarboxylic acids, for example cyclic neopentyl glycol adipate, butanediol adipate, hexanediol adipate, hexanediol sebacate, etc.
U.S. Pat. No. 5,712,230, for example, claims low-emission polyester polyurethane foams in which polyesters wherein the monomers cannot form any cycles having ring sizes between 12 and 14 atoms are used.
WO 2012125353 claims polyester polyols based on phthalic anhydride and selected diol units, for example ethylene glycol, propylene glycol, neopentyl glycol or hexanediol. The polyester polyols do have a low concentration of cyclic dimers, but the polymers are not crystalline.